Diesel engines fall into two main categories, namely heavy duty, being principally large engines for trucks, buses and prime mover vehicle engines, ships and boats and stationary engines, and light duty engines, used in smaller trucks and cars. With the increasingly demanding regulation of emissions from all sorts of engines, attention is now being paid to control of emissions such as particulates and NOx from diesel engines. We have, in EP 341,832, described a device marketed as the Continuously Regenerating Trap. (“CRT™”) by Johnson Matthey. This device traps sooty particulates and causes their combustion by exposing them to NO2 generally generated by catalytically oxidising NO present in the exhaust stream. This device has met with considerable success in controlling particulate emissions from heavy duty diesel engines, and can achieve zero emissions of particulate. That is, the CRT as marketed is approximately 100% effective to remove particulates (as defined in the regulations).
We have realised that a conventional CRT may not be cost-effective to control emissions to European Stage III, IV, or higher, regulations from light duty engines. A number of different options for controlling particulates and NOx are available, and engine manufacturers have hitherto favoured engine design and management solutions. In general, engine design itself can satisfy European Stage II emissions standards as regards NOx and particulates, but the characteristics of the diesel engine are such that engine design cannot improve upon about 0.4 g/Km particulates without a serious increase in NOx, which is difficult to deal with under lean conditions. There still remains, therefore, a need for effective systems to meet these emissions regulations whilst increasing engine design options.
EP 0759713 (Toyota) describes an addition to the CRT principles, involving the absorption in a NOx absorbent of NO from gases leaving the CRT-type combination of oxidising catalyst and diesel particulate filter or trap, formed by one of the reactions of NO2 with carbon particles.